Rate enhancements of SN2 reactions of methyl naphthalene-2-sulfonate by sulfobetaine micelles

Abstract

Rate constants of S_N2 reactions of methyl naphthalene-2-sulfonate (MeONs) with nucleophilic anions and amines are increased by zwitterionic micelles of tetradecyl-dimethyl-, -dipropyl- and -dibutyl-ammoniopropanesulfonate (SB3-14, SBPr3-14 and SBBu3-14, respectively) when anions are Cl⁻, N₃⁻, CN⁻, HCO₂⁻ and S₂O₃²⁻ and the amines are n-propylamine, di-n-propylamine, n-butylamine and quinuclidine. On the other hand, reactions of OH⁻, SO₃²⁻ and morpholine are inhibited but not suppressed. The rate effects are fitted quantitatively by a pseudophase model that considers the transfer equilibria of both reactants between water and micelles and second-order rate constants in each pseudophase. Second-order rate constants for the ionic reactions in the micellar pseudophase are similar to those in water and, where data are available, to those in cationic micelles. The second-order rate constants for reactions of the amines are lower than those in water. The increased head group bulk in SBBu3-14 over SB3-14 decreases the polarity in the micellar interfacial region favoring reactions of soft anions and disfavoring those of hard ions and of amines.